Q: Many electric underground metro trains can accelerate rapidly from a standing start at a platform. Why are most electric trains running above ground apparently unable to achieve the same rapid acceleration?
A: When deep sections of the London Underground were being constructed, the track at the stations was built at a higher level than the track between the stations. Trains leaving a station travel downhill, which reduces rolling friction and increases acceleration. Conversely, trains approaching a station can be decelerated by running up a slope, thereby reducing brake wear. This system is obviously ideal when trains stop at every station and was easy to construct while tunnelling was being carried out. Of course, it would be impractical on surface lines.
A: Electric passenger trains have only a single mechanical gear ratio. The ratio is chosen to suit the application, so underground trains with frequent stops and low speeds are provided with a low gear ratio, whereas surface trains are higher geared, which gives low acceleration and higher maximum speed. Consequently, intercity trains have the highest gearing.
Advertisement
A: Two main factors allow underground trains to achieve higher acceleration than their surface counterparts.
Rails are smooth, giving low rolling losses, but their smoothness makes friction at the wheel/rail interface a limiting factor. A train cannot achieve acceleration greater than the proportion of its weight carried on its powered wheels multiplied by the coefficient of friction between wheel and rail, unless traction systems such as rack and pinion are used.
The underground environment keeps the rails comparatively clean and dry, so an underground railway can rely on a considerably higher coefficient of friction between the driven wheels and the rails than a surface railway. Typical coefficients of friction are 0.2 underground (0.12 minimum) and 0.1 on a good day above ground (0.08 minimum), falling to about half that in severe weather. The motors are set to achieve the minimum expected figure so wheel spin (which damages rails and wastes time) is just avoided.
This advantage over surface trains is not sufficient for the underground metro operator. A metro makes frequent stops, so its journey times depend largely on the acceleration which it can achieve. Metros commonly fit motors to two thirds of the train wheels. Long distance trains have only a quarter or so of the wheels powered, to save money.
Therefore an underground metro train can typically accelerate at about 0.67 脳 0.12g = 0.08g, but a long distance train can only achieve about 0.25 脳 0.08g = 0.02g on a good day.
A: It is likely the question refers to underground trains which have a substantial feeder rail, and to surface trains or elevated trains with an overhead cable feed. If so, the underground train can receive a heavier current for a short time, giving the greater initial acceleration.
A: There is an illusion of speed created by the confined area of a tunnel. While seemingly catapulting through the bowels of the city, underground trains in central London rarely reach 30 kilometres per hour.